Structures prone to be stroked by lightning, such as wind turbine blades, are typically equipped with a lightning conductor to conduct at least most of the current of a lightning through or past the structure to the ground without damaging sensitive parts of the structure.
If other conducting material than the lightning conductor is present in the structure, flashover between the other conducting material and the lightning conductor may take place if the potential difference between the lightning conductor and the other conducting material becomes sufficiently great. Providing a means for potential equalising may prevent this or at least the risk of flashover should be greatly reduced.
Means for potential equalising typically consist of one or more electrically conducting cords between the lightning conductor and the other conducting material. However, when the other conducting material is not a very good conductor in one or more directions, a sufficiently good potential equalising to prevent flashover cannot be realised directly by an electrically conducting cord. This is particularly the case for wind turbine blades comprising carbon fibres, e.g. as reinforcement fibres.
Carbon fibres in a composite material will typically conduct electricity considerably worse orthogonal to the length of the fibres compared to parallel to the length of the fibres. This is mainly due to relatively high contact resistance and low contact area between the fibres orthogonal to the length direction. To realise potential equalising, a good electrical contact needs to be established between the members to be equalised. Due to the nature of the carbon-fibres, this may typically not be realised with a point contact to an electrical cable.
Generally speaking only an infinitesimal current needs to be transferred to realise the potential equalising. On the other hand, in the case of conduction of lightning in or near a wind turbine blade, conducting parts, which may be hit directly by lightning, should be connected to a lightning conductor with potential equalising members, which are capable of transferring the current of a lightning. The current of a lightning may be greater than 100 kA in short periods of time, i.e. a completely different situation compared to the transport of the infinitesimal currents needed for potential equalising.